Mo. Rev. Stat. § 78.220
No ordinance passed by the council, except when otherwise required by the general laws of the state, or by the provisions of sections 78.010 to 78.420, except an ordinance for the immediate preservation of the public peace, health or safety, which contains a statement of its urgency and is passed by a two-thirds vote of the council shall go into effect before ten days from the time of its final passage; and if during said ten days a petition signed by the voters of the city equal in number to at least twenty-five percent of the entire vote cast for all candidates for mayor at the last preceding municipal election at which a mayor was elected, protesting against the passage of such ordinance be presented to the council, the same shall thereupon be suspended from going into operation; and it shall be the duty of the council to reconsider such ordinance, and if the same is not entirely repealed, the council shall submit the ordinance as is provided by subdivision (2) of section 78.200, to the vote of the voters. Such ordinance shall not go into effect or become operative unless a majority of the voters voting on the same shall vote in favor thereof. Said petition shall be in all respects in accordance with the provisions of said sections 78.260 to 78.290 except as to the percentage of signers, and be examined and certified to by the clerk in all respects as therein provided.
§ 78.220, RSMo